Ocean Chemistry and
Composition
Brian Schuster
Chemical Properties of Sea Water
• polar: unequal sharing of electrons
• hydrogen bonding: intermolecular dipole-dipole
interaction between H2O molecules
• cohesion: sticking of H2O molecules to each other
resulting from hydrogen bonding; causes surface tension
• adhesion: sticking of H2O molecules to other polar
materials
• capillarity: movement of water up a small tube
• latent heat: energy stored in water that doesn’t change
it’s temperature
Acidity/Alkalinity
•
•
•
ocean: pH 7.5 to 8.5 due to carbonate (CO3-2)
from dissociation of calcium carbonate
freshwater: pH 6.5 to 7.5 due to carbonic acid
(H2CO3)
bicarbonate buffering, resistance to pH change
–
–
–
H2O + CO2  H2CO3 (carbonic acid=weak acid)
H2CO3  H+ + HCO3- (bicarbonate=buffer)
CaCO3  Ca+2 + CO3-2 (carbonate=base)
Temperature
• depth dependent
• thermocline: rapid
change in temperature
around a certain
depth; more defined
near equator; less
prominent at poles
• isotherm: line of
constant temperature
Global isotherms display currents
Salinity
• isohaline: line of constant salinity
• residence time: average time a molecule spends in
a certain reservoir
• salinometers: determine salinity through
conductivity
• halocline: rapid change in salinity
• constancy of composition: major ion constituents
exist in constant proportions
• water is the “universal solvent”
• ocean salinity: 35ppt, brackish: 17ppt, brine: >50ppt
• saltiest sea: Dead Sea
Salinity (continued)
• Long-term sources:
– hydrothermal vents & volcanoes
– weathering of rocks
• cation: positively charged ion
• anion: negatively charged ion
• salinity effected by:
– precipitation, evaporation
– freezing, thawing
– river input
• Top six constituents
–
–
–
–
–
–
chlorine (Cl-) 55%
sodium (Na+) 31%
sulfate (SO4-2) 8%
magnesium (Mg+2) 4%
calcium (Ca+2) 1%
potassium (K+) 1%
Desalination
1.
2.
3.
4.
reverse osmosis
freezing & thawing
ion exchange
distillation (evaporation & condensation)
Density
• pycnocline: rapid change in density
• freshwater most dense at 4 °C, saltwater most
dense just before freezing
• saltwater freezes at -2 °C (freezing point
depression)
• density affected by:
– temperature
– salinity
– depth, pressure: minimally
• isopycnal: surface of constant density
Pressure and Gases
• rises by 1 atm (14.7 psi, 101.3 kPa) for
every 10 m (33 ft) depth
• absolute pressure at 20 m is 3 atm
• gauge pressure at 20 m is 2 atm
• isobar: line of constant pressure
Dissolved Gases
• Henry’s Law: more
gas can be dissolved
under high pressure
• more gas is
dissolved in deep,
cold water
• oxygen minimum
zone: depth depends
on productivity and
aerobic respiration
• atmospheric gases
resemble ocean’s
due to mixing
Calcium Carbonate (CaCO3)
• used in shells, compound of limestone and
calcite, base of coral reef
• sources and sinks
– chemical precipitation, dissolution
– weathering of limestone and calcite
– organisms
• solubility increased by:
– greater depths
– higher acidity (lower pH)
• carbonate compensation depth (CCD):
dissolution rate of CaCO3 equals supply rate
Nutrients
• eutrophication: excessive
addition of nutrients
• oligotrophication:
excessive removal of
nutrients
• iron is the limiting nutrient
for most algae
South Atlantic
phytoplankton bloom
Heat Transfer
• conduction: molecules speed each other
up by physically bumping
• convection: carried by movement of fluid
• radiation: electromagnetic radiation is
absorbed and raises temperature
• heat is conducted faster in water than in
air, so divers can get cold faster
Sound
• in water, speed of sound = ~1500 m/s
(3500 mph), 5x speed in air
• speed changes with density
• sound travels farther in water than light, so
it is good for cetacean communication
• SOFAR (sound fixing and ranging)
channel: sound travels slower around
1000 m, and can go farther; sound gets
stuck in this channel
Light
• penetration
– long wavelengths (red) absorbed first
– mid-range wavelengths (green, blue) go farthest
• attenuation: decrease in light intensity due to
absorption and scattering by suspended particles
• turbidity increases attenuation
• index of refraction (n) = 1.33
Other
• conservative property: mostly affected by mixing and
diffusion (ex: salinity)
• non-conservative property: affected mostly by
processes other than mixing and diffusion (ex:
dissolved oxygen relating to productivity)
• temperature-salinity diagram: unique to different
bodies of water; shows lines of constant density in
sigma units